I found an issue in overlapped.c: read_buffer and write_buffer attributes are defined in an union, but they are sometimes use even if the type is not good. I found this issue while trying to get a BrokenPipeError on Windows when the stdin stream of a subprocess is full but the process exited.

Could you please review attached patch, especially the change in Overlapped_getresult?

Another option would be to use two real attributes (remove the union), but I like the usage of union :-)

All Tulip unit tests of my subprocess_stream pass on Linux and Windows with the patch.